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H¬2O2
is an environmentally-benign and selective oxidant useful for epoxidations and
bleaching, yet, its use is limited because the anthraquinone oxidation process
is viable only at very large scales. Currently, many oxidation processes use
chlorine-based oxidants (Cl2, HOCl) at a global rate of 63·109 kg Cl yr-1.
Within the United States roughly 60% of chlorine is used as an oxidant in
processes where chlorine does not appear in the final products (e.g.,
production of propylene oxide). The direct synthesis of H2O2 (H2 + O2 ®
H2O2) could enable H2O2 to be produced cost-effectively on-site, and even in
situ, which would curtail needless use of chlorine, and in turn, decrease
environmental chlorine pollution. This seminar will describe findings based on
rate measurements, isotope effects, and in situ spectroscopy that reveal
mechanisms, active intermediates, site requirements, and functional descriptors
for the formation of H2O2 and its use in epoxidations.
First, direct synthesis of H2O2 involves chemistry at the solid-liquid
interface between the complex “broths” needed for high selectivities and
supported monometallic (Pd) or bimetallic clusters. Our finding suggest that
solvent molecules themselves (e.g., methanol) act as co-catalysts and likely
facilitate unexpected proton-electron transfer processes that form H2O2 while
surface reactions that cleave O-O bonds produce water. The rates of these two
pathways differ dramatically in their sensitivity on the composition of the solvent
and the catalyst surface, which provides opportunities for developing highly
selective catalytic systems.
Second, transition metal atoms contained in the framework of zeolites or
grafted to the surface of mesoporous oxides are used to epoxidize olefins with
H2O2. The identities and reactivities of the oxidizing surface species that
form (i.e., –OOH or –O2) upon irreversible activation of H2O2 on these
catalysts depend on the identity and intrinsic properties of the metal
substituents. Among only Group 4 and 5 transition metals, measurable increases
in the electrophilicity of metal centers lead to epoxidation rates that
increase over five-orders of magnitude and which are accompanied by a
concomitant 100-fold increase in selectivities. Within the micropores of
zeolites, dispersive interactions with the surroundings of active sites can be
used to preferentially stabilize reactive states that lead to epoxidations, and
therefore, increase selectivities to epoxides and their rates of formation.
In ongoing work, we aim to develop guiding principles and
reactivity/selectivity descriptors for the design of metallic catalysts for the
direct synthesis of H2O2 and subsequent selective oxidations by peroxo and
hydroperoxo intermediates.